1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet, which is a recording medium, for use in a printer, a copying machine, a facsimile machine, or the like, and particularly to an image forming apparatus for forming an image on a sheet by moving a recording head, which is image formation means, for scanning while conveying the sheet.
2. Related Background Art
In general, there is widely known an image forming apparatus comprising a recording medium separating and feeding means for separating and feeding a sheaf of sheets one by one, a recording medium conveying means for conveying the separated and fed sheet, an image forming means for forming an image on the sheet under conveyance, and a recording medium discharging means for discharging the sheet on which the image formation has been completed to an outside of the apparatus; it is used as a printer, a copying machine, or a facsimile machine, for example.
Referring to FIG. 4, there is shown a perspective diagram illustrating a structure of an ink-jet printer, given as an example of this type of image forming apparatus. Referring to FIG. 5, there is shown a cross section of the structure of the ink-jet printer. Referring to FIG. 6, there is shown a diagram illustrating a transmission configuration for conveying a sheet in the image forming apparatus.
As shown in FIG. 4 and FIG. 5, the ink-jet printer comprises a feeder 1 (i.e. a paper feeding apparatus), a platen 6, a stepping motor gear 7 (FIG. 5), a conveying roller 8, a sensor lever 9 (FIG. 5), a sheet sensor 10 (FIG. 5), a pinch roller spring 11 (FIG. 5), a pinch roller 12, a recording head 13 as image forming means, a stepping motor 14, a conveying roller gear 15, a carriage 16, a guide shaft 17, a guide rail 18 (FIG. 5), a carriage motor 19, a timing belt 20, discharging rollers 21, follower spurs 22, a frame 23 (FIG. 5), intermediate gears 24, and discharging roller gears 25.
The feeder 1, which is a recording medium separating and feeding means, comprises a plate 2, a feeder base 3, a feeding motor 4, a feeding roller 5, and a plate spring 27. The recording medium conveying means is a conveying roller pair comprising the conveying roller 8 and the pinch roller 12. The recording medium discharging means is a discharging roller pair comprising the discharging rollers 21 and the follower spurs 22. A recording medium detecting means comprises the sensor lever 9 and the sheet sensor 10. The frame 23 is used for mounting the above composing elements.
In addition, as shown in FIG. 6, the stepping motor gear 7 arranged at a rotary shaft of the stepping motor 14 is coupled to the conveying roller gear 15 arranged at the conveying roller 8, two intermediate gears 24, and two discharging roller gears arranged at the discharging rollers 21 in a sheet conveying direction. Therefore, the conveying roller 8 is driven by the stepping motor 14, which is a driving source, via the stepping motor gear 7 and the conveying roller gear 15. The discharging rollers 21 are driven by the stepping motor 14, which is a driving source, via the stepping motor gear 7, the conveying roller gear 15, the intermediate gears 24, and the discharging roller gears 25.
As shown in FIG. 5, the plate 2 is supported by the feeder base 3 in a condition that it can turn, and the plate 2 is stacked with a sheaf of sheets on its top surface. For sheet feeding, the feeding motor 4, which is a driving source, rotates the feeding roller 5 and the plate 2 turns toward the feeding roller 5 by means of the plate spring 24, and the sheaf of sheets are put in contact with the feeding roller 5 with pressure.
Furthermore, when the feeding roller 5 rotates, only the top sheet among the sheets is separated from the sheaf of the sheets and then fed to the downstream. The sheet separated and fed by the feeder 1 is fed to the conveying roller pair by a further rotation of the feeding roller 5. Then, a leading edge of the sheet pushes the sensor lever 9 arranged between the feeding roller 5 and the conveying roller 8 so as to turn the sensor lever 9. The sheet sensor 10 detects that the leading edge of the sheet has reached a given position from the sensor lever 9 turning and falling out of the inside of the sheet sensor 10 and detects that a trailing edge of the sheet has reached a given position from the sensor lever 9 entering into the sheet sensor 10 again.
If the sheet sensor 10 detects the leading edge of the sheet, the sheet is conveyed by a given amount by the feeding roller 5 and then abutted against a nip portion 26 between the conveying roller 8 and the pinch roller 12 urged to the conveying roller 8 by means of the pinch roller spring 11. When the feeding roller 5 further conveys the sheet by a given amount in this condition, the leading edge of the sheet is pushed to the nip portion 26 and the registration is completed.
After the completion of the registration, the sheet is conveyed to the top of the platen 6 by a rotation of the conveying roller 8 and supported by the top surface of the platen 6 at a position where the sheet is opposing to an arrangement surface of an ink discharge nozzle of the recording head 13.
Subsequently, ink drops are discharged from the recording head 13 mounted on the carriage 16 for scanning the sheet supported by the top surface of the platen, by which an image is formed on the sheet. The carriage 16 is supported by the guide shaft 17 and the guide rail 18 in a condition that scanning is possible and is driven by the carriage motor 19 via the timing belt 20.
The sheet on which the image formation has completed is discharged to an outside of the apparatus by rotations of the discharging rollers 21 and the follower spurs 22 urged to the discharging rollers 21 by means of spur springs (not shown).
The following summarizes a flow of conveying a sheet in the ink-jet printer with reference to FIG. 7. As shown in FIG. 7, the sheet fed by the feeding roller 5 is conveyed by a rotation of the conveying roller pair comprising the conveying roller 8 and the pinch roller 12, which is its follower roller, to a position where an image is formed by the recording head 13. Then, the image is formed on the sheet through a repetition of scanning with the carriage 16 having the recording head 13 and sheet conveyance with the conveying roller pair.
The sheet on which the image formation has completed is discharged to the outside of the image forming apparatus by two discharging roller pairs comprising the discharging rollers 21 and their follower rollers, the follower spurs 22. As described above, the sheet sensor 10 detects that the leading edge or the trailing edge of the sheet has reached the given position and the stepping motor 14 drives both of the conveying roller pair and the discharging roller pairs.
Conventionally, the image forming apparatus considers a load on the stepping motor 14 or torque characteristics before applying a sine-curved driving current having a fixed peak value as shown in FIG. 8 or a rectangular-waveform driving current to the stepping motor 14 so as to control the rotation of the stepping motor 14.
This type of image forming apparatuses include one having a configuration in which the image formation is further continued after the trailing edge of the sheet passes the conveying roller pair. In this image forming apparatus, a sheet conveying state changes from a conveying state with the conveying roller pair or with both of the conveying roller pair and the discharging roller pairs (hereinafter, referred to as a conveying state with the conveying roller pair) to a conveying state with the discharging roller pairs only in the middle of the image formation.
In this image forming apparatus, generally the urging force of the pinch roller 12 at the conveying roller pair is greater than the urging force of the follower spurs 22 at the discharging roller pairs and therefore the load on the stepping motor 14 in the conveying state with the conveying roller pair is greater than that in the conveying state with the discharging roller pairs only. Even if, however, the load on the stepping motor 14 becomes smaller in the conveying state with the discharging roller pairs only, the peak value of the driving current applied to the stepping motor 14 remains at the fixed level. This causes a problem that the stepping motor 14 has excessive torque, thereby generating larger driving noise of the stepping motor 14 or a problem that an overshoot rises at a stop of the stepping motor 14, thereby deteriorating a sheet conveying accuracy.
On the other hand, it is known in this image forming apparatus that the overshoot at the stop of the stepping motor 14 drops by continuously applying phase-excitation, namely, a fixed amount of electric current to the stepping motor 14 to retain its stop phase when the stepping motor 14 is stopped during the image formation, by which the stopping accuracy of the stepping motor 14 is improved.
If, however, a fixed amount of electric current is continuously applied to the stepping motor 14 to retain its stop phase when the stepping motor 14 is stopped during the image formation, there is a problem that the stepping motor 14 generates large stop noise at the moment the stepping motor 14 stops.
An experiment proved that this noise in the conveying state with the conveying roller pair is larger than the noise in the conveying state with the discharging roller pairs only. As described above, the load on the stepping motor 14 in the conveying state with the discharging roller pairs only is smaller than in the conveying state with the conveying roller pair and therefore there is a large effect of retaining the stop phase by applying a fixed amount of electric current. In the conveying state with the conveying roller pair, however, a large load is applied on the stepping motor 14 and therefore the overshoot at the stop of the stepping motor 14 is low without retaining the stop phase by applying a fixed amount of electric current, by which an experiment proved that a sufficient conveying accuracy is ensured.
As set forth in the above, the conventional image forming apparatus has the following problems:
(1) The load on the stepping motor is large in the conveying state with the conveying roller pair in comparison with the load in the conveying state with the discharging roller pairs only. Even if the load on the stepping motor drops in the conveying state with the discharging roller pairs only, however, the peak value of the electric current applied to the stepping motor remains at the fixed level. This causes problems that the stepping motor has redundant torque, thereby causing large driving noise of the stepping motor or that an overshoot amount rises at the stop of the stepping motor, thereby deteriorating the conveying accuracy of the sheet.
(2) When the stepping motor is stopped during the image formation, a fixed amount of electric current is continuously applied to the stepping motor to retain its stop phase, which decreases an overshoot amount at the stop of the stepping motor and improves the stopping accuracy of the stepping motor. If this method is used, however, there is a problem that the stepping motor generates large stop noise.
Therefore it is an object of the present invention to provide an image forming apparatus which can prevent deterioration of a stopping accuracy caused by an overshoot at a stop of a stepping motor and can reduce noise generated by the stepping motor.
It is another object of the present invention to provide an image forming apparatus for forming an image on a recording medium with a recording head, comprising a conveying roller pair for conveying the recording medium, discharging roller pairs for conveying the recording medium in the downstream of the conveying roller pair, and a stepping motor for driving the conveying roller pair and the discharging roller pairs, wherein a peak value of driving current of the stepping motor after a trailing edge of the recording medium passes the conveying roller pair is smaller than a peak value of driving current of the stepping motor before the trailing edge of the recording medium passes the conveying roller pair.
It is still another object of the present invention to provide an image forming apparatus for forming an image on a recording medium with a recording head, comprising a conveying roller pair for conveying the recording medium, discharging roller pairs for conveying the recording medium in the downstream of the conveying roller pair, and a stepping motor for driving the conveying roller pair and the discharging roller pairs, wherein phase-excitation of the stepping motor is conducted before a trailing edge of the recording medium reaches the conveying roller pair and the phase-excitation of the stepping motor is halted after the trailing edge of the recording medium passes the conveying roller pair.
It is a further object of the present invention to provide an image forming apparatus for forming an image on a recording medium with a recording head, comprising a conveying roller pair for conveying the recording medium, discharging roller pairs for conveying the recording medium in the downstream of the conveying roller pair, and a stepping motor for driving the conveying roller pair and the discharging roller pairs, wherein a peak value of driving current of the stepping motor after a trailing edge of the recording medium passes the conveying roller pair is smaller than a peak value of driving current of the stepping motor before the trailing edge of the recording medium passes the conveying roller pair and wherein phase-excitation of the stepping motor is performed before the trailing edge of the recording medium passes the conveying roller pair and the phase-excitation of the stepping motor is halted after the trailing edge of the recording medium passes the conveying roller pair.